1. Field of the Invention
The invention relates generally to signal receivers having in-phase (I) and quadrature phase (Q) signal processing and more particularly to methods and apparatus for balancing I/Q gain and I/Q phase in a signal receiver.
2. Description of the Prior Art
Most modern radio signal receivers estimate the data that was transmitted by processing in-phase (I) and quadrature phase (Q) signal components. The I and Q signals should have a phase difference (I/Q phase) of 90° at the carrier frequency of the incoming signal and a gain ratio (I/Q gain) of unity. However imperfections in the analog circuitry used in the radio frequency (RF) quadrature downconverters in most modern signal receivers cause the I/Q gain and I/Q phase to be out of balance (I/Q gain not equal to one and I/Q phase not equal to 90°). These imbalances cause a degradation in bit error rate (BER) in estimating the transmitted data.
Existing signal receivers use several methods for correcting I/Q gain and I/Q phase imbalances within the receivers. In one method, an offline test signal is used during manufacture or installation to align the I/Q gain to unity and the I/Q phase to 90° in the signal receiver. However, the performance of the receivers using the test signal method is limited by drift in the analog circuitry after the alignment. This limitation is reduced by performing the alignment periodically during operation. However, the periodic alignment adds overhead that reduces the efficiency of a signal communication channel.
A second method uses an adaptive algorithm that processes the I and Q signals for converging to adjustments to the I and Q signals while the receiver is on-line. However, the BER performance of the receivers using the adaptive algorithm method is degraded because the receiver is estimating the transmitted data during the same on-line time period that the adaptive algorithm is converging. Of course, the adaptive algorithm could be performed on a test signal but this would add overhead and reduce signal efficiency.
Existing receivers using the test signal method or the adaptive algorithm method sometimes use correction coefficients for balancing I/Q gain and I/Q phase of the I and Q signals. However, such receivers that are known determine the I/Q gain and the I/Q phase corrections at points in the signal path that are separated from the RF quadrature downconverter by subsequent downconversion and/or demodulation of the I and Q signals. The performance of such receivers is limited because the imbalances are converted to image signals by the downconversion and/or demodulation and the degradation effect of such image signals cannot be completely eliminated once they are formed.
There is a need for a method for correcting I/Q gain and I/Q phase imbalance in a signal receiver without adding overhead to the signal communication channel and without degrading BER while converging on correction coefficients.